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HETEROKARYOSIS has generally been considered an important component of the genetic systems of fungi (Pontecorvo, 1946; Raper, 1955; Buxton, 1960). This conclusion should be questioned, however, in view of the demonstration in recent years of systems of heterokaryon or vegetative incompatibility which restrict heterokaryon formation to strains of like genotype, and thus greatly reduce the genetic significance of heterokaryosis (seeCaten and Jinks, 1966, for review). The evolutionary consequences of restrictions on heterokaryon formation will be most marked in imperfect species, where heterokaryosis coupled with parasexuality (Pontecorvo, 1956) provides the only known method of recombination. However, the detailed investigations of heterokaryon formation between unrelated strains which led to the discovery of heterokaryon incompatibility all involved perfect species (Garnjobst, 1953, 1955; Beisson-Schecroun, 1962; Grindle, 1963a, b; Jones, 1965; Esser, 1966; Kwon and Raper, 1967), and no comparable studies with imperfect species have been reported. In view of the added significance of heterokaryosis in the absence of sexual reproduction, it seemed likely that heterokaryon incompatibility might be either absent or less extensive in imperfect forms. Gossop, Yuill and YuilI (1940) reported the absence of heterokaryotic conidial heads in mixtures of unrelated strains (inter-strain combinations) of Aspergillus niger, but gave no information concerning the number of mixtures examined. A similar result was obtained by Raper and Fennell (1953) in two out of three inter-strain combinations of A. fonsecaeus, but the interpretation was complicated by the failure of certain combinations of mutants from the same parent (intra-strain combinations) to produce heterokaryotic heads. Jinks (personal communication) failed to synthesise heterokaryons by pairing unrelated, but morphologically similar, English strains of the Asymmetrica group of the genus Penicilliurn, despite the fact that similar attempts to reconstitute naturally occurring heterokaryons from their component homokaryons were successful. These brief reports suggest that heterokaryon incompatibility does occur in at least two groups of imperfect fungi, but allow no quantitative statements. The present work was undertaken to extend the investigations of heterokaryon formation between unrelated strains to further imperfect species, and to provide estimates of the extent of heterokaryon incompatibility which could be compared with those previously reported for related perfect species. The production of mixed conidial heads (Gossop et al., 1940) in mixed cultures of wild-type and conidial colour mutants has many advantages as a criterion of heterokaryon formation for a study of this type (Grindle, 1963 a). It has one major disadvantage, however, in that it demands a species which produces well-defined conidial heads and in which the conidia are coloured

Heterokaryon incompatibility in imperfect species of Aspergillus